The present invention relates to a video signal processing apparatus that limits the level of a received video signal when the level of the received video signal has exceeded a predetermined level by a noise or interference.
When a video signal is transmitted from an imaging apparatus or an image reproducing apparatus to a receiver for image reproduction, the video signal is generally transmitted in the form of an analog video signal or a digital video signal. During such a transmission of the video signal, there can be a case in which the level of the video signal exceeds a predetermined level for reception as a result of mixing of noise or interference with other signals including other video signals. When the level of the received video signal exceeds the predetermined level as such, there can be a case in which a pseudo-synchronization is caused. Alternatively, the excessive level of the video signal may cause a deterioration in the quality of reproduced image. Thus, there is a need to limit the level of a received video signal within a predetermined level.
FIG. 1A shows a conventional image signal transmission system implemented in the form of a coded image transmission system, wherein the video signal transmission system includes an analog video signal output unit 301, which may be an imaging device or a video signal recording/reproducing apparatus, an encoding and transmission unit 302 for encoding and transmitting the analog video signal from the unit 301 over a transmission path 305, a video signal processing unit 303 receiving the encoded video signal from the unit 302 through the transmission path 305, a video signal processing unit 303 decoding the encoded video signal thus received and converting the same into an analog video signal, and a reproducing unit 304 for reproducing the analog video signal thus recovered by the video signal processing unit 303. As can be seen in FIG. 1A, the encoding and transmission unit 302 includes an A-D converter 306 for converting the input analog video signal to a digital video signal, an encoder 307 for encoding the digital video signal produced by the A-D converter 306 and an interface device 308 for transmitting the encoded video signal over the transmission path 305. Further, the video signal processing unit 303 includes an interface unit 309 for receiving the encoded video signal from the transmission path 305, a decoder 310 for decoding the encoded video signal thus received by the interface unit 309, and a D-A converter 311 for converting the decoded video signal to an analog video signal.
FIG. 1B shows another conventional image signal transmission system implemented in the form of a digital image transmission system, wherein the video signal transmission system includes an analog video signal output unit 321, which may be an imaging device or a video signal recording/reproducing apparatus, a digital transmission unit 322 for converting the analog video signal from the unit 321 into a digital video signal and transmitting the same over a transmission path 325, a video signal processing unit 323 receiving the digital video signal from the digital transmission unit 322 through the transmission path 305, a video signal processing unit 323 converting the received digital video signal into an analog video signal, and a reproducing unit 324 for reproducing the analog video signal thus recovered by the video signal processing unit 323. As can be seen in FIG. 1B, the transmission unit 322 includes an A-D converter 326 for converting the input analog video signal to the digital video signal and an interface device 327 for transmitting the digital video signal over the transmission path 325. Further, the video signal processing unit 323 includes an interface unit 328 for receiving the digital video signal from the transmission path 325 and a D-A converter 329 for converting the digital video signal to an analog video signal.
FIG. 1C shows another conventional image signal transmission system implemented in the form of an analog image transmission system, wherein the video signal transmission system includes an analog video signal output unit 331, which may be an imaging device or a video signal recording/reproducing apparatus, an analog transmission unit 332 for transmitting the analog video signal from the unit 331 over a transmission path 335, a video signal processing unit 333 receiving the analog video signal from the analog transmission unit 332 through the transmission path 335, the video signal processing unit 333 receiving the an analog video signal from the transmission unit 332 via the transmission path 335, and a reproducing unit 334 for reproducing the analog video signal thus received by the video signal processing unit 333. As can be seen in FIG. 1C, the transmission unit 332 includes an A-D converter 336 for converting the input analog video signal to the digital video signal, while the video signal processing unit 333 includes an interface unit 337 for receiving the analog video signal from the transmission path 335.
FIG. 2A shows a 100% color bar signal that includes a synchronization pulse of -40 (IRE) level and a video signal, wherein the video signal has a level not lower than the level of the synchronization signal. The maximum level of the video signal is set to 133 (IRE) level.
When the video signal has a level lower than the -40 (IRE) level as indicated in FIG. 2B, it will be noted that the portion of the video signal having such a low level state is erroneously recognized as being a synchronization pulse, and there arises a problem of erroneous pseudo-synchronization. When the video signal includes a portion having a level exceeding 133 (IRE), on the other hand, the quality of the reproduced image is deteriorated as a result of saturation.
In the encoded video signal transmission system of FIG. 1A, it should be noted that the A-D converter 306 and the D-A converter 311 create error in the video signal associated with analog-to-digital conversion or digital-to-analog conversion of the video signal. Further, the encoder 307 and the decoder 310 create error as a result of the encoding or decoding of the video signal. Further, the transmission path 325 may pickup noise.
In the digital video signal transmission system of FIG. 1B, the A-D converter 326 and the D-A converter 329 create error in the video signal associated with the analog-to-digital conversion or digital-to-analog conversion of the video signal. Further, the transmission path 305 may pickup noise. In the analog video signal transmission system of FIG. 1C, on the other hand, the transmission path 335 may pickup noise.
As a result of the mixing of noise or conversion error, or as a result of the interference of the signals transmitted in a multiplexed mode, there can be a case in which the video signal supplied from the video signal processing unit 303, 323 or 333 to the reproducing unit 304, 324 or 334 contains an under-level portion or over-level portion as indicated in FIG. 2B. When this is the case, the under-level portion of the video signal may be erroneously recognized as being a synchronization pulse, and there occurs a problem of pseudo-synchronization. When the video signal contains an over-level portion, on the other hand, the quality of the reproduced image may be deteriorated.